A fan shroud that supports an axial fan cooling a radiator of a vehicle is described in Patent Document 1. The fan shroud has a part in which a gap in a radial direction between an outer rim of the fan shroud and a ring portion surrounding the axial fan in a radially outer side is large, and a part in which the gap is small. An air guide portion is provided between an outer rim of the fan shroud and the ring portion. Areas of the air guide portion corresponding to an upper part and a lower part of the fan shroud are small, and areas of the air guide portion corresponding to a left part and a right part are large, for example.
In recent years, due to high requirement of quietness in a vehicle, peak sounds regarding a rotation noise of the fan such as peak noises of 1-order and n-order is required to be reduced. The rotation noise is remarkably increased by interference of a rotating body and air around the rotating body, and single frequency element becomes particularly high sound pressure.
When a large area portion and a small area portion are provided in the air guide portion of the fan shroud as described in Patent Document 1, a velocity of a main air flow flowing to an inside of the ring portion in a direction along a rotation axis when a blade of the fan passes the large area portion is remarkably different form the velocity of the main air flow when the blade passes the small area portion. In the large area portion of the air guide portion, the main air flow flows in a direction inclined at a large angle with respect to the rotation axis. In the small area portion of the air guide portion, the main air flow flows in a direction slightly inclined with respect to the rotation axis. Therefore, the velocity of the main air flow in the direction along the rotation axis, i.e. a velocity vector in the direction along the rotation axis is large in the small area portion.
Since a pressure difference between an upstream side and a downstream side of the blade of the fan is generated, a counter air flow flowing toward the upstream side along the rotation axis is generated in the downstream side of the blade. A collision of the counter air flow and the main air flow causes a swirl. Since the velocity of the main air flow in the direction along the rotation axis is large around the small area portion of the air guide portion, the swirl is likely to be generated on the downstream side of the blade around the small area portion. In contrast, around the large area portion, since the velocity of the main air flow in the direction along the rotation axis is small, the swirl is likely to be generated on the upstream side of the blade. Accordingly, effects of the swirl on the fan shroud around the large area portion are small, and effects of the swirl on the fan shroud around the small area portion are large. Accordingly, around the small area portion, a negative pressure area is generated on the surface of the shroud due to an interference of the swirl, and a peak sound of n-order increases when the blade passes the small area portion, and accordingly a rotation noise may be generated.